Loudspeakers are intended to accurately reproduce sound over the full spectrum of the audio range. The full spectrum of musical sound covers a range from about 20-30 Hertz (Hz) in the bass to about 14,000-20,000 Hz or so in the treble. This is the range in which most instruments produce the fundamental tones and combination of overtones that give them their distinctive sound. A measure of a loudspeaker's accuracy of sound reproduction is its ability to change electrical input signals into sound that corresponds almost exactly to those input signals. A loudspeaker should not alter or color the sound intended, but this is a common problem with present loudspeakers. For example, different loudspeakers can have similar accuracy of response over the audio frequency range, but they can sound entirely different from one another. This indicates that the sound reproduction is being colored by the different loudspeaker designs.
Great care is taken in the modern recording industry to cause the sound recorded on a phonograph record or take to correspond, in frequency and volume, as faithfully as possible to the sound of the performance reproduced in the recording. Similarly, modern electronic audio equipment (amplifiers and the like) are extremely linear over their operating ranges, and they faithfully amplify and generate output signals that also faithfully correspond to the sound in the recording. A loudspeaker system used to transduce the electrical output of the audio amplifier to an audible signal should not color the sound, since this subverts the care taken in the original recording and in the reproduction amplifiers.
Resonances and vibrations in the loudspeaker system color the sound intended. I have discovered that the structure of the loudspeaker enclosure itself is a principal source of resonances and vibrations in the loudspeaker system. Sound coloration can be caused by objectional resonances of the loudspeaker enclosure at one or more frequencies within the frequency range of their operation. Loudspeaker resonances result in boominess of the speaker, which is not consistent with faithful reproduction of the sound intended. Sound coloration also can be caused by physical motion or vibration transferred to the walls of the speaker enclosure from vibrations induced in the loudspeaker during operation.
A need therefore exists for an improved loudspeaker enclosure which, when used in combination with a loudspeaker, reproduces sound over the intended frequency range without objectionable resonances and without significant physical motion or vibration being transmitted to the walls of the speaker enclosure from vibrations of the loudspeaker.
My U.S. Pat. No. 4,139,076 meets these needs by providing a loudspeaker enclosure with improved freedom from resonance. The loudspeaker enclosure in my patent includes a housing having a principal volume and substantially smaller minor volume. The minor volume is provided by a small chamber that projects essentially entirely beyond the front wall of the principal volume. The front wall of the small chamber provides a speaker mounting opening. To the extent that my earlier patent requires the small chamber to project entirely beyond the front wall of the principal volume, it has since been recognized that this is a special case of a broader development. It has been discovered that the benefits of my developments in loudspeaker enclosures can be realized by enclosures having other configurations, and the purpose of this application is to disclose configurations which comprise this broader development.